I'm bored and having nothing better to do, so I will bite... I can go on and on and on about my problems with Wicked.
My biggest problem with Wicked is that is did not stay more true to Maguire's novel which I found to be a literary masterpiece. Intense, substantial, dark, vivid, striking, and beautiful. The musical is NONE of these things. It took Maguire's concept and title, and created a "bubblegum" and "cotton candy" teeny bopper theme park musical with a lousy score and terrible book. The ending of the show is ridiculous and basically just took a massive SH*T on the theme and message Maguire so eloquently wove into the novel. I honestly do not even think the musical should have been allowed to have the same title as the book.
My other problem is the atrocious design elements that clash and create a mismatched train wreck that is not pretty to look at it. Let's start with the clock theme. The whole concept for the design is based off the Clock of the Time Dragon - something that was VERY pivotal to the plot of the book. The Clock of the Time Dragon is referenced maybe two or three times during the show, but apparently it makes sense to have it act as THE central concept and heart of the set design. The cogs, gears, and metal are just plain ugly IMO... Then there's the costumes that look like they were ripped out of a Dr. Seuss book. The costumes just don't work, and are way too over the top... Again, this is all my opinion. Wicked is one of the shows that LOOKS expensive. You can look up at the stage and see it is a big high-quality intricate production. However, I have never seen a show look so expensive and plain ugly at the same time.
Then there's the book and score. If the musical had stayed more true to the book, this show could have been something extraordinary and groundbreaking instead of just a mega theme park show holding court at the Gershwin. The musical could have/should have been MUCH darker. The book and score are too fluffy for my liking, especially considering the dark and deep messages, themes, and tones in the original book.
So that's why I hate Wicked. It's an entertaining show, but I find it to be one of the most overhyped and ugliest shows I have seen. It's a mash of fluffy nonsense mixed with some ugly design elements and underdeveloped characters and plot points.
Again, this all my opinion... The OP asked why all the hate, and there they have it.
"There’s nothing quite like the power and the passion of Broadway music. "